1. Field of the Invention
The invention relates to a liquid crystal display device, and more particularly, to a liquid crystal display device including a light-emitting diode (LED) light source.
2. Discussion of the Related Art
Liquid crystal display (LCD) devices are most widely used for monitors of notebook computers, monitors of personal computers and televisions due to excellent moving images and high contrast ratio. LCD devices use the optical anisotropy and polarization properties of liquid crystal molecules of a liquid crystal layer to produce an image.
An LCD device includes two substrates spaced apart and facing each other and a liquid crystal layer interposed between the two substrates. The alignment direction of the liquid crystal molecules is controlled by varying the intensity of an electric field applied to the liquid crystal layer, and the transmittance of light through the liquid crystal layer is changed.
The LCD devices require an additional light source because the LCD devices are not self-luminescent. Therefore, a backlight unit is disposed at a rear side of a liquid crystal (LC) panel and emits light into the LC panel, whereby discernible images can be displayed.
Backlight units include cold cathode fluorescent lamps (CCFLs), external electrode fluorescent lamps (EEFLs), and light emitting diodes (LEDs) as a light source. Among these, LED lamps have been widely used due to their small sizes, low power consumption, and high reliability.
FIG. 1 is a cross-sectional view illustrating a liquid crystal display (LCD) module including LEDs as a light source according to the related art.
In FIG. 1, the related art LCD module includes a liquid crystal panel 10, a backlight unit 20, a support main 30, a top cover 40 and a cover bottom 50.
The liquid crystal panel 10 displays images and includes first and second substrates 12 and 14 facing and attached to each other with a liquid crystal layer (not shown) interposed therebetween. Polarizers 19a and 19b are attached at front and rear surfaces of the liquid crystal panel 10 and control the polarization of light.
The backlight unit 20 is disposed at a rear side of the liquid crystal panel 10. The backlight unit 20 includes an LED assembly 29, a reflection sheet 25, a light guide plate 23 and a plurality of optical sheets 21. The LED assembly 29 is disposed at an edge of at least one side of the support main 30 along a length direction. The reflection sheet 25 is disposed over the cover bottom 50 and is white- or silver-colored. The light guide plate 23 is disposed over the reflection sheet 25. The plurality of optical sheets 21 are disposed over the light guide plate 23.
The LED assembly 29 is disposed at a side of the light guide plate 23. The LED assembly 29 includes a plurality of LEDs 29a emitting white light and a printed circuit board (PCB) 29b on which the LEDs 29a are mounted.
Edges of the liquid crystal panel 10 and the backlight unit 20 are surrounded by the support main 30 having a rectangular frame shape. The top cover 40 covers edges of the front surface of the liquid crystal panel 10, and the cover bottom 50 covers a rear surface of the backlight unit 20. The top cover 40 and the cover bottom 50 are combined to with the support main 30 to thereby constitute one-united body.
FIG. 2 is a cross-sectional view of enlarging an area A of FIG. 1. In FIG. 2, the LEDs 29a are arranged along the side of the light guide plate 23 of the LCD module, and the LEDs 29a are mounted on the PCB 29b to constitute the LED assembly 29. The LED assembly 29 is fixed by a bonding method such that lights emitted from the LEDs 29a face a side surface of the light guide plate 23, which the lights are incident on and which is referred to as a light-incident surface hereinafter. To do this, the cover bottom 50 has a side wall that is formed by bending an edge portion of the cover bottom 50 upward. The LED assembly 29 is attached to the side wall of the cover bottom 50 by an adhesive material such as a both-sided sticky tape. The structure may be referred to as a side top view type.
Accordingly, lights emitted from the LEDs 29a are incident on the light-incident surface of the light guide plate 23 and then are refracted toward the liquid crystal panel 10 inside the light guide plate 23. With lights reflected by the reflection sheet 25, the lights are changed to have uniform brightness and high qualities through the plurality of optical sheets 21 and are provided to the liquid crystal panel 10. Accordingly, the liquid crystal panel 10 displays images.
However, there are several problems in the LCD device including the backlight unit 20. Specially, the LCD device does not include a guiding means such that all the lights emitted from the LEDs 29a go to the light guide plate 23. Thus, some of lights emitted from are lost between the LED 29a and the light guide plate 23, and there is loss of light in the LCD device.
FIG. 3 is a view of showing a simulation result of changes in luminous flux after lights emitted from the LEDs 29a of FIG. 2 are changed into a plane light source by the light guide plate 23 of FIG. 2 according to the related art. Here, the unit of luminous flux is lm (lumen) in the International System of Units. As the luminous flux gets high, the quantity of light also gets high.
In the simulation result, as it goes from green to red, the luminous flux gets high. That is, there is no luminous flux in green, and the luminous flux increases as it goes from yellow to red.
Here, an ideal change in the luminous flux is as follows. There is no luminous flux in a side edge portion of the LCD device, where the LEDs 29a of FIG. 2 are disposed and which is a non-display area excluding pixels, and thus the side edge portion is shown as green. In the other portions of the LCD device, where the lights emitted from the LEDs 29a of FIG. 2 are incident on the light guide plate 23 of FIG. 2 and changed into the plane light source, there is high luminous flux, and thus the other portions are shown as red.
However, according to FIG. 3, there exists yellow between green and red. In yellow, the luminous flux is lower than red, and there is the loss of light. Here, yellow corresponds to a region between the LEDs 29a of FIG. 2 and the light-incident surface of the light guide plate 23 of FIG. 2. Accordingly, it is found that there occurs the loss of light between the LEDs and the light guide plate.
When a total luminous flux of the lights emitted from the LEDs 29a of FIG. 2 is 100 lm, the plane light source from the light guide plate 23 of FIG. 2 has the luminous flux of 79.74 lm. Therefore, the loss of the luminous flux may be about 20 lm.
This causes lowering of the qualities of the LCD device such as the brightness and image qualities.